(1) Field of the Invention
The present invention relates to a control system for a flapping foil maneuvering device used on an underwater vehicle in which there is a plurality of the flapping foil devices positioned on the vehicle. The motion of the flapping foils is enabled by using non-linear auto-catalytic oscillators.
(2) Description of the Prior Art
A comparison of underwater vehicles and swimming animals, in the context of cruising and maneuvering, illustrates a divergence in their performance. While underwater vehicles outperform their biological counterparts when it comes to cruising, maneuvering (especially at low-speeds) is achieved more efficiently by swimming animals.
The maneuverability of swimming animals is facilitated by the use of foils extending from the body of the animal in which the foil-type appendages flap almost continuously. From this basic configuration, animal-like agility and flexible use of different swim modes are possible for the vehicle by selection of the type, number, and location of the foils.
Swimming animals accomplish low-speed maneuverability by unsteady actuation of the foils. The unsteady actuation of the foils allows the generation of high lift at the foils via dynamic stall and torque production (even at low and zero forward speeds), which leads to efficiency and high maneuverability. A relatively high lift coefficient can be achieved by using unsteady hydrodynamics as compared to using steady hydrodynamics.
Swimming animals are also more capable of maneuvering with a small turn radius although the gap between animals and man-made devices in maneuvering has narrowed due to improvements in digital controllers. In a further effort to narrow or bridge this gap, high-lift principles observed in swimming and flying animals have been implemented in an underwater vehicle. As a result, the underwater vehicle has three animal-like features; the first feature is low speed maneuverability, the second feature is low noise production and the third feature is high efficiency in the form of low power consumption.
The value of neuroscience-based control of the underwater vehicle is evident when non-linear auto-catalytic properties of oscillators are used to introduce local autonomy; thereby, providing the underwater vehicle with a natural robust property (without relying on sensors) when responding to unforeseen disturbances and obstructions. Similar to the non-linear mechanisms that an Inferior-Olive system uses to produce a robust balancing motion in animals, non-linear oscillators flap the propulsive foils of the underwater vehicle to produce a robust and balanced locomotion of the underwater vehicle.
More specifically and relevant to the disclosure that follows, an underwater vehicle developed by the United States Navy has multiple flapping foils positioned on an exterior of the vehicle. Each of the flapping foils can independently execute a pitching and rolling motion. The flapping foils also have multiple degrees of freedom that can input periodic 3-D forces and moments, whose amplitude, frequency, phase and relative bias can be independently controlled. Data gathered from the undersea vehicle indicates that with high-lift actuators and appropriate controllers, it is possible to achieve a maneuvering optimization that is similar to animals and even superior to the maneuverability of animals.
The motion of the flapping foils allows the generation of high lift via dynamic stall and torque production, which leads to efficiency and high maneuverability. A continuous and controlled flapping motion of these foils has been documented to produce the necessary lift and thrust and the ability to produce various maneuvers of the undersea vehicle at different speeds.
With the appropriate actuation that can deliver the requisite mobility, a critical area of support for actuation is the calculation and processing of an efficient swimming algorithm. The first requirement of actuation is the property of robustness, which is the ability to deliver a periodic motion in the presence of environmental and system disturbances. The second requirement of actuation is that the controls occur with local autonomy by preferably a sensor-less configuration.
A method for generating such periodic motion with sensor-less autonomy and robustness is by using a non-linear system that is capable of producing self-excited oscillations and having the ability to maintain an oscillatory mode despite perturbations. While a periodic motion such as a sinusoid can be derived using linear circuits, the robustness property is not inherently present and can be incorporated only by using sensor-based feedback and suitable compensation to the feedback.
Non-linear actuators do exist in nature and are ubiquitous in living organisms. For example, circadian rhythms, cardiac rhythms, hormonal cycles, rhythms of breathing and swimming, the stable orbits of astronomical bodies, are all produced in nature. Periodic oscillations of a specified magnitude and that are frequency-synthesized using non-linear differential equations are referred to as limit cycles. The limit cycle properties of non-linear oscillators have been observed to be present in many branches of natural science including biology.
Unlike linear systems that exhibit sustained oscillations whose amplitude is proportional to the magnitude of the disturbances that the systems are subjected to, limit cycles have the ability to maintain a prescribed profile of oscillations and the ability to return to the prescribed profile even when disturbed and once a disturbance is removed. It could therefore be argued that instances of sustained oscillations observed in nature are necessarily produced by such limit cycles, that the oscillations are non-linear and are auto-catalytic. This self-organizing nature is perhaps the reason that biological phenomena provide a baseline for generating rhythmic patterns. Such exemplified structural stability makes a limit cycle a reasonable option for engineering design.
It is also known in biology that central pattern generators abound in a central nervous system and are routinely used for synchronizing the motor actions of limbs during locomotion. In the context of locomotion, proper execution of active movements in animals appears to occur as a function of the olivo-cerebellar system of the brain. An olivo-cerebellar system is an autonomous system of neurons that can generate a rhythmic pattern of neuronal discharge that can ultimately coordinate muscles in a manner similar to that seen during normal locomotion. This concept in general, and the specific fact that non-linear oscillators are adept at producing robust periodic orbits have been studied extensively in the context of walking by humans and robots.
The utilization of non-linear oscillators by the inferior olive system provides an inherent robustness to the generation of synchronized signals thereby providing signals to the appropriate motor neurons and therefore providing an operating robustness to the intended movement.
As a result, an opportunity exists to use nature-like features of maneuvering by utilization of auto-catalytic non-linear oscillators to drive the flapping foils of an underwater vehicle so as to achieve an operating robustness.